A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
A semiconductor substrate on which the IC is manufactured typically exhibit some variation of the surface level: the surface of a substrate is typically not flat. Typically, the surface level varies as a function of the position on the substrate around an average level. To some extent, such surface level variations are induced by the manufacturing process of the substrate. One of the components of the surface level variation can be characterized as a (local) curvature of the surface. In practice, local surface level variations due to curvature may be several tens of nanometers.
In particular, for lithographic apparatus that can achieve critical dimension values of the same order as the local curvature, a compensation for this curvature may be needed. Due to the relatively small focal depth of the projection system of such a lithographic apparatus, a local deviation between the image plane including the image and the surface level due to curvature may result in a local defocus. As a consequence, the pattern as created on the surface may be inaccurate with regard to the size and/or position of features of the pattern.
Due to the fact that the image created by the projection system, has a fixed shape (i.e., it is usually calibrated towards a planar surface), the exposure on the substrate with curvature or varying curvature as a function of position on the substrate will result in a pattern image of which the focus quality may vary as a function of the position within the image. In particular, edges of substrate may suffer from curvature (variations) which may affect the possibility to print a pattern near those edges of the substrate.
In a scanner lithographic system as described above, the pattern image is created during scanning by imaging a section of the patterning device (e.g. mask) onto the substrate. The projected image of this section, here referred to as the exposure slit has a rectangular shape. The width of the slit (that is in the direction perpendicular to the scanning direction) corresponds to the width of an exposure field in which the pattern is imaged. The length of the slit in the scanning direction is typically much shorter than the width of the slit. For this reason, the largest effect of curvature on defocus may occur in the non-scanning direction.
Defocus due to curvature along the scanning direction may, to some extent, be compensated by leveling the substrate relative to the image plane of the projection system as a function of the position of the slit on the substrate. However, in the direction perpendicular to the scanning direction the curvature may be relatively large without the possibility of correction.